Yes, and that works - when I run the package from the command line. But how do I apply this to a .MSI package that is supposed to be distributed via AD GPO only? Is there anyway to add parameters to an existing MSI package in a software policy? Or am I missing the point completley? [&:]

To assign this property a value in a GPO deployment, I would create an MST (Transform) and create and assign this property to it. Then in the Advanced deployment properties you can use the "modifications" tab to specify the MST file. I believe the Microsoft Office Custom Installation Wizard (CIW) it the preferred way to generate an MST for Visio. You may still need to edit the resutling MST to add a value for this property after you have created the MST- if you do not see it as an option in the CIW.

You may not be able to modify the vendor's MSI but if you create a transform you should be able to add the property to the Property table and save it. I created a transform using PackageStudio and then added the ACCEPTEULA=1 property. Running the MSI with the transform worked great.

I did this with Wise Package Studio 4.62 and 5.1.
All you need to do is open the msi file and go to the Execute Immediate script and go to
the section with Cancel Installation [EULAErrorText] [ConfirmEULA] and Comment that line and the End line out with a "*/". You will also need to do this in any Transforms that you may be using with the msi file.